The present invention relates to the valve control of a hydraulic actuator, wherein multiple, adjustable control positions are provided for the selective control of passage and blocking of the flow of hydraulic fluid through supply and return ducts.
The utilization of more or less complex control valves controlling a hydraulic actuator entails that, from time to time, certain maintenance work has to be performed. Such valves are, for example, used in the aircraft industry, and maintenance here is, of course, required for reasons of safety and has to be performed regularly and periodically in order to ensure that the aircraft is consistently operational in all of its functions. It is obvious that components requiring such maintenance and here particularly control valves in the hydraulic system should be constructed to permit such maintenance with ease. Moreover, it has to be observed that the valves are part of a, more or less, complex hydraulic system; after the maintenance work on the valves has been completed, the hydraulic system has to be vented and recharged with hydraulic fluid, but any accidental inclusion, for example, of air bubbles in the hydraulic fluid has to be avoided positively as that may have fatal effects.
Considering the foregoing requirements, it has been customary in the aircraft industry to provide particularly located and constructed venting valves within the hydraulic actuator system of the aircraft. These venting valves are located between the outputs of regular control valves and the hydraulic actuator to be operated by the control valves. These suppplemental venting valves do, indeed, ensure the complete removal of air bubbles from the hydraulic system. On the other hand, the expenditure as such is too high, i.e., it is deemed undesirable to provide a separate valve just for purposes of venting the system! Not only do such venting valves constitute additional components, they add also weight; and of course, they have their own maintenance requirements. For this reason, a valve has been constructed in which a regular control valve is modified to include in a separate and additional operational state for the venting function. It was found, however, that in practice this type of valve posed problems because maintenance people may not always exercise due care in the adjustment of the valve concerning the venting disposition. Therefore, it has been found in practice that, even though a valve was presumed to have been placed in a venting position, the venting function had not been completely performed and that, after maintenance was supposedly completed, air bubbles were still included in the hydraulic system, particularly air bubbles trapped in relatively high-positioned components.